Regional blood flow to the brain changes with neural activity. We want to measure directly the blood flow in cerebral microvessels in the whisker barrel cortex of mice and rats. We are testing the hypothesis that the initial vasodilation and increased blood flow in the cerebral vasculature during natural activation of the brain is prompt and is localized in a small volume of the brain. The barrel cortex provides an ideal context for these studies because its neurophysiology, neuroanatomy, metabolism, and vascular architecture are well known. Video microscopy with image analysis will be used to record and measure the velocities of intravascular fluorescent beads and vessel diameters with fluorescent plasma markers. Laser Doppler flowmetry will provide convenient independent estimates of changes in local blood volume, velocity, and flow. Both assays will be used to evaluate the properties of the cortical vascular bed before, during, and after stimulation of the contralateral whiskers.